videoconvert: Filter-out crop meta

To passthrough crop-meta, the converter would need to allocate and
convert buffers of the size of the originating buffer. This is currently
made difficult by GstBaseTransform since we cannot alter the caps passed
though the allocation query. We would also need to wait for the first
input buffer to be received in order to make the decision around that
size.
So the short and safe solution is just to stop pretending we can
passthrought that meta.
https://bugzilla.gnome.org/show_bug.cgi?id=791412